Electrical apparatus for transcutaneously (through the skin) stimulating nerve and/or muscle tissues in the human body are generally well known, heretofore primarily for therapy or to control pain. In therapy for injured muscles, for instance, electrical signals are applied so as to contract and relax the muscle to bring in nutrients and release toxins therefrom, thereby promoting healing of the muscle.
Such electrical apparatus has also been used to maintain or enchance muscle tone, as well as muscle size and strength. Many of these devices use what is referred to hereinafter as a sine wave pulse signal to transmit electrical energy to the bodily area of interest. The term sine wave pulse, as used herein, is a pulse having positive and negative excursions about a zero voltage line. A sine wave pulse may be in the form of a sine wave, as that term is commonly understood, or it may comprise a square wave, or any other configuration, so long as the signal includes successive positive and negative going excursions, or vice versa, centered substantially about a zero volt reference line.
In this category of electrical transcutaneous devices, it is well known to control both the pulse rate, i.e. the rate at which the sine wave pulses occur, as well as the duration or width of each pulse. Both of these parameters may typically be varied within a selected range available on the device itself, particularly in pain control devices which have varying optimal operating conditions, depending upon the user and the level of pain encountered. The device may also have fixed, i.e. non-variable, values of pulse rate and width in order to provide particular therapeutic results.
In many cases, however, particularly in therapeutic situations, optimum results are not obtained, because the discomfort caused by the electrical signals results in an increase in muscle tension in the body, interfering with the therapeutic process. Heretofore, a significant reduction of optimum therapeutic results must be accepted in order to achieve reasonable or acceptable comfort levels.
The discomfort to the patient is caused by irritation to the skin as well as irritation to the muscle or nerve. Skin irritation is typically due to the electrical resistance of the skin. The skin will readily and typically painlessly pass pulses with therapeutically significant voltage levels having relatively short pulse widths (100 microseconds or less). However, the longer the pulse width, with other factors such as voltage level remaining the same, the more resistive the skin generally tends to act, and skin irritation increases accordingly, producing an unpleasant stinging effect. The effect will vary from patient to patient, depending upon the sensitivity of each patient. Also, at high pulse rates (100-1000 pps), painful tetany is experienced under the electrodes.
Muscle irritation is dependent upon pulse rate and pulse width. Typically, the lower the pulse rate, the farther the contractile effect penetrates into the muscle. Lower pulse rates (4-25 pps) produce a sudden "pulling and dropping" effect in which the muscle contracts too fast or too hard. The longer the pulse width, the more pronounced is the effect.
There is often an imbalance of contractions which is experienced by the user, because the muscle under the one electrode that initially goes positive before transitioning negatively will contract more strongly than the muscle under the electrode that initially goes negative before transitioning positively. This is apparently due to the fact that muscles react to change in the voltage applied and that a negative-going change has a significantly greater effect than a corresponding positive-going change. Hence, the muscles closest to the area of negative stimulus contract more strongly than the muscles closest to the area of positive stimulus. The amount or strength of the stimulus will depend on the overall change in the voltage applied, i.e. from +25 v to -25 v, rather than the actual level per se of the voltage. Thus, specifically for sine wave pulses, the muscles closest to the area of stimulus receiving a negative-going signal from an initially positive signal level will experience a greater contraction than those muscles closest to the area of stimulus receiving a positive-going signal from an initial negative signal level.
It is also known that an ion transfer phenomenon occurs in tissue when electrical pulses are applied thereto. When the positive-going and negative-going pulses are electrically balanced, a net ion transfer does not occur, but the muscle contraction feels unbalanced, due to the contraction effect discussed immediately above. When the muscle contraction is balanced by unbalancing the positive- and negative-going pulses, the now stronger positive pulse causes an ion transfer which is not fully compensated for by the negative pulse. This net ion transfer can result in discomfort and/or a chemical change in the tissue under the electrodes, effects which are generally regarded as undesirable in sine pulse therapy.
Therefore, in summary, in conventional sine wave therapy, either a contractile imbalance or a net ion transfer occurs.
In another approach, using electrical pulses in a particular manner referred to as DIC (dual inferential currents), the above-described problems with skin irritation and electrical (contractile) imbalance are eliminated. With the DIC approach, four electrodes are used with two high frequency signals. The frequencies of the two signals are slightly different, and where the two signal paths intersect, at the bodily area of treatment interest, there is a nulling effect, and the effective signal frequency at the treatment area is the difference between the frequencies of the two individual signals. The disadvantage of this approach, however, is the cost of the equipment and the set-up time required, as well as the necessity of a trained operator.
The present invention, on the other hand, is directed toward a transcutaneous apparatus which produces a sine pulse train which overcomes the disadvantages of the prior art and produces a high therapeutic effect for selected nerve/muscle tissue with minimal resulting discomfort.